Implications of rRNA Operon Copy Number and Ribosome Content in the Marine Oligotrophic Ultramicrobacterium Sphingomonas sp. Strain RB2256

Abstract

Sphingomonas sp. strain RB2256 is a representative of the dominant class of ultramicrobacteria that are present in marine oligotrophic waters. In this study we examined the rRNA copy number and ribosome content of RB2256 to identify factors that may be associated with the relatively low rate of growth exhibited by the organism. It was found that RB2256 contains a single copy of the rRNA operon, in contrast to Vibrio spp., which contain more than eight copies. The maximum number of ribosomes per cell was observed during mid-log phase; however, this maximum content was low compared to those of faster-growing, heterotrophic bacteria (approximately 8% of the maximum ribosome content of Escherichia coli with a growth rate of 1.5 h−1). The low number of ribosomes per cell appears to correlate with the low rate of growth (0.16 to 0.18 h−1) and the presence of a single copy of the rRNA operon. However, on the basis of cell volume, RB2256 appears to have a higher concentration of ribosomes than E. coli (approximately double that of E. coli with a growth rate of 1.5 h−1). Ribosome numbers reached maximum levels during mid-log-phase growth but decreased rapidly to 10% of maximum during late log phase through 7 days of starvation. The cells in late log phase and at the onset of starvation displayed an immediate response to a sudden addition of excess glucose (3 mM). This result demonstrates that a ribosome content 10% of maximum is sufficient to allow cells to immediately respond to nutrient upshift and achieve maximum rates of growth. These data indicate that the bulk of the ribosome pool is not required for protein synthesis and that ribosomes are not the limiting factor contributing to a low rate of growth. Our findings show that the regulation of ribosome content, the number of ribosomes per cell, and growth rate responses in RB2256 are fundamentally different from those characteristics in fast-growing heterotrophs like E. coliand that they may be characteristics typical of oligotrophic ultramicrobacteria.

Bacterial diversity of metagenomic and PCR libraries from the Delaware River

“…Eighty clones tested positive for 16S rRNA genes, which is consistent with the expected number of rRNA genes in the library, assuming a single 16S rRNA gene per bacterial genome. Bacteria isolated from aquatic and low-nutrient environments typically have only one or two rRNA operons (Button et al ., 1998;Fegatella et al ., 1998;Fogel et al ., 1999;Klappenbach et al ., 2000).…”

Ribosomal RNA content in microcolony forming soil bacteria measured by quantitative 16S rRNA hybridization and image analysis

“…Alcaligenes A2 had though a lower growth rate on the 1/10 strength TSB medium than P. £uorescens R2f and Bacillus B1 which may also explain the low rRNA concentration in Alcaligenes A2. While a close relation between balanced growth, cell volume and rRNA concentration has been established for a number of bacteria [1^4,7] the coupling to the number of rrna operons in the bacteria has only been studied in an oligotrophic ultramicrobacterium carrying a single rrna operon [6]. This bacterium had a low number of ribosomes compared to E. coli carrying seven rrna operons, but in terms of rRNA concentration this was much higher in the ultramicrobacterium due to its small cell volume.…”

“…This has been used to estimate in situ activity of Pseudomonas putida in a toluene degrading bio-reactor [5]. However, di¡erent bac-teria may independent of their growth rates express di¡erent concentrations of ribosomes [4,6,7]. Thus, the rRNA concentration depends not only on growth rates but seems to be under a di¡erent regulation in di¡erent species.…”

“…Thus, the rRNA concentration depends not only on growth rates but seems to be under a di¡erent regulation in di¡erent species. It has been shown that a slow growing oligotrophic ultramicrobacterium carried only a single copy of the rrna operon [6], while other more mesotrophic bacteria such as Bacillus cereus may have more than 12 copies of the operon [8]. Recently Klappenbach et al [9] found a correlation between the variation in rrna operon multiplicity and the capacity of bacteria to respond to addition of nutrients.…”

“…For example an oligotrophic microbacterium Sphingomonas sp. having only a single rrna operon had in total fewer ribosomes than E. coli, but the concentration turned out to be much higher due to the small cell volume [6]. A relationship between growth rates, rRNA concentration and the number of rrna operons has not been studied so far for bacteria representing a wider range of rrna operon numbers.…”